MySQL Cookbook

Preventing Duplicates from Occurring in a Table

Problem

You want to prevent a table from ever containing duplicates, so that you won’t have to worry about eliminating them later.

Solution

Use a PRIMARY KEY or a UNIQUE index.

Discussion

To make sure that records in a table are unique, some column or combination of columns must be required to contain unique values in each row. When this requirement is satisfied, you can refer to any record in the table unambiguously using its unique identifier. To make sure a table has this characteristic, include a PRIMARY KEY or UNIQUE index in the table structure when you create the table. The following table contains no such index, so it would allow duplicate records:

CREATE TABLE person
(
    last_name   CHAR(20),
    first_name  CHAR(20),
    address     CHAR(40)
);

To prevent multiple records with the same first and last name values from being created in this table, add a PRIMARY KEY to its definition. When you do this, it’s also necessary to declare the indexed columns to be NOT NULL, because a PRIMARY KEY does not allow NULL values:

CREATE TABLE person
(
    last_name   CHAR(20) NOT NULL,
    first_name  CHAR(20) NOT NULL,
    address     CHAR(40),
    PRIMARY KEY (last_name, first_name)
);

The presence of a unique index in a table normally causes an error to occur if you insert a record into the table that duplicates an existing record in the column or columns that define the index. Recipe 14.3 discusses how to handle such errors or modify MySQL’s duplicate-handling behavior.

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